1. Field of the Invention
The present invention relates to a magneto-optical recording medium having an information recording layer and a reproducing layer, together with a method of recording and reproducing the same, and an apparatus for recording and reproducing it. More specifically, it relates to a magneto-optical recording medium capable of generating a clock which is precisely synchronised with a recording domain recorded on the information recording layer, together with a method of recording and reproducing the same, and an apparatus for recording and reproducing it.
2. Description of the Related Art
Magneto-optical recording media and other magneto-optical discs are familiar as external memory in computers and elsewhere. Magneto-optical recording media permit the rewriting of information, and their capacity to handle large volumes of moving image, sound and other data has led to their frequent use as recording media in this multimedia ages. In recent years there have been calls for further increases in the storage capacity of such magneto-optical recording media, and one method of achieving this would be to make the recording magnetic domains even smaller and record information at higher densities. Recording with smaller recording magnetic domains is possible if a light pulse magnetic field modulation process is adopted whereby a magnetic field of a polarity depending on the recording signal is applied while irradiating with light which has been modulated into pulse form in synchrony with the recording clock.
However, any attempt to generate magnetic microdomains encounters the problem that it is impossible to make the spot diameter of the reproducing light smaller than the NA limit of the light head, and there is no way of reproducing separately a plurality of magnetic microdomains existing within the reproducing light spot. One suggested method of solving this problem is by means of the technique of magnetic super-resolution (MSR) (Journal of the Magnetic society of Japan, vol. 17, suppl. S1, p. 201, 1993). According to this technique, it is possible to reproduce one of two recording magnetic domains existing within a reproducing light spot by masking the other so that it cannot be seen, thus narrowing the effective field of vision. However, the strength of the reproduction signal from each of the magnetic domains remains the same, with the result that the C/N is low.
In International Patent Application WO98/02878 the inventors of the present invention have disclosed a magneto-optical recording medium which has a magnetic domain magnification reproducing layer and a recording layer on the substrates making it possible to transfer the magnetic microdomains of the recording layer separately at reproduction, and by applying a reproducing electric field to enlarge the magnetic domains which have been transferred to the recording layer prior to reproducing them. This magneto-optical recording medium facilitates a marked increase in the strength of the reproduction signal because the magnetic domains which are transferred on to the magnetic domain magnification reproducing layer are enlarged (magnified) to roughly light spot size. The technique has been given the name of MAMMOS (Magnetic Amplifying Magneto-Optical System), and serves to solve the problems inherent in the above-mentioned magnetic super-resolution technique in relation to the reproduction S/N of magnetic microdomains.
In MAMMOS, not only do the magnetic domains recorded on the recording layer need to be enlarged at the moment when they are transferred to the reproducing layer, but the enlarged magnetic domains must be erased immediately before the next recording magnetic domain is scanned by the light spot. For this reason, it is necessary to control the timing with which the external magnetic field is applied and the polarity reversed. MAMMOS is in the process of becoming a very important technique for improving recording density, but on the other hand certain problems become evident as the density increases.
The process whereby information is recorded on to a magneto-optical recording medium involves either light modulation, electric field modulation or a combination of the two. Whatever process is adopted, it is normal to detect a pre-pit formed on the substrate, thus generating a clock, which then becomes the criterion for irradiating with light and/or applying an external magnetic field, in accordance with the information which is to be recorded.
In the above-mentioned method of recording, however, the recording layer is irradiated with recording light and heated to a prescribed temperature while the recording electric field is applied. This is then cooled to form recording magnetic domains on the recording layer Thus, depending on conditions such as the strength of the recording light and the material from which the recording layer is fashioned, it may happen that a delay occurs before the recording magnetic domains are actually formed on the recording layer, causing them to diverge from their prescribed positions according to the recording clock. On the other hand, the fact that the reproducing external magnetic field in MAMMOS is also based on a clock pit which is formed on the substrate means there is no guarantee that the external magnetic field is being applied in line with positions where recording magnetic domains actually exist. For this reason it proved to be no easy matter to adjust the modulation timing of the reproducing external magnetic field and reproducing light applied to a MAMMOS magneto-optical recording medium.
It is an object of the present invention, which has been designed to eliminate the problems inherent in the related art, to provide a magneto-optical recording medium capable of generating a reproducing clock in synchrony to a high degree of precision with the recording domain which is recorded on the information recording layer, and to provide a method of recording and reproducing same, and apparatus for recording and reproducing it. It is a further object of the present invention to provide a magneto-optical recording medium which permits easy adjustment of the optimum timing for applying reproducing light and reproducing electric field when reproducing a magneto-optical recording medium of the type wherein a reproducing magnetic field is applied along with the reproducing light, and to provide a method of recording and reproducing same, and apparatus for recording and reproducing it.
The first aspect of the present invention provides a magneto-optical recording medium comprising an information recording layer on which information is recorded; and a reproducing layer on to which information recorded on the information recording layer is transferred, a clock mark being formed on the information recording layer.
The magneto-optical recording medium according to the present invention has clock marks formed on the information recording layer along with information recording marks. This allows a reproducing clock to be generated in accordance with the clock marks. The following are two examples of methods which may be employed in order to detect the clock marks recorded on the information recording layer,
The first method of detection involves the use of two types of light of differing wavelengths xcex1 and xcex2. The reproducing layer is reproduced by means of light of wavelength xcex1, while light of wavelength xcex2 is used to read the clock marks recorded on the recording layer. The reason for using light of a different wavelength to read the reproducing clock marks is as follows. In MAMMOS and MSR there are reproducing and intermediate layers above the recording layer, so that light absorption by these layers serves to diminish the reproducing signal from the clock marks of the recording layer. What is more, the need to acquire the reproducing clocks in advance of information reproduction makes it difficult to use information reproducing light to reproduce the clock marks. The use of light of a different wavelength from that used for information reproduction makes it possible to read without difficulty the clock marks which are recorded on the information recording layer. A clock generated in accordance with clock marks read in this manner can be employed for the purpose of modulating the reproducing light to pulse form prior to irradiation, or modulating and applying the reproducing magnetic field used in AMOS and MSR.
The second method of detection consists in detecting the clock pit formed on the substrate, while at the same time irradiating the magneto-optical recording medium with light and applying a direct-current magnetic field of the same polarity as the clock marks formed on the information recording layer. Applying a direct-current magnetic field along with the incident light means that the magnetisation (magnetic domain) of the clock marks recorded on the information recording layer by virtue of this direct-current magnetic field is transferred on to the reproducing layer, the coercive force of which has been diminished by the incident light. Once the reproduction signal based on a clock mark has been detected from the reproducing layer, application of the direct-current magnetic field is halted, and a clock is generated in accordance with the detected reproduction signal. This second method of detection differs from the first in that it is possible to read the clock marks and generate a clock without employing two types of light of differing frequencies. The clock thus generated can be employed, in the same way as one generated by the first method of detection, for the purpose of modulating the reproducing light to pulse form prior to irradiation, or modulating and applying the reproducing magnetic field used in MAMMOS and MSR. The clock marks formed on the information recording layer of the magneto-optical recording medium according to the present invention function as timing marks for optimising or synchronising the application timing of the reproducing light and reproducing magnetic field with respect to the positions of the recording marks.
The reproducing layer of the magneto-optical recording medium according to the present invention should preferably be such that the magnetic domain transferred from the information recording layer is enlarged prior to reproduction by applying it with an external magnetic field of the same polarity as the magnetisation of the magnetic domain. In other words, the fact that the present invention makes it possible to generate a reproducing clock from the clock marks formed on the information recording layer means that it is possible, by applying in synchrony with this reproducing clock a magnetic field in order to enlarge the magnetic domain, not only accurately to transfer a recording magnetic domain which has diverged from the recording clock, but also to enlarge it and reproduce it at a higher C/N. What is more, in the second method of detection described above it is possible to reproduce at a higher C/N by enlarging by means of a magnetic domain magnification reproducing layer the clock marks recorded on the information recording layer. This means that it is possible accurately to generate the reproducing clock signal even if the clock marks are small.
Moreover, it should be remembered that in the present invention the light is incident from the side where the reproducing layer is formed. It is therefore preferable when in the first method of detection described above light of wavelength xcex2 is employed in order to detect the clock marks recorded on the information recording layer, either to select as wavelength xcex2 one which passes through the reproducing layer or to adjust the material of the reproducing layer so that light of wavelength xcex2 passes through it. It is also preferable for clock marks recorded on the information recording layer to be distinguished from recording marks corresponding to recording information by being longer than the shortest recording mark.
When recording marks corresponding to the desired recording information on the information recording layer in the magneto-optical recording medium to which the present invention pertains, it is also possible to do so by processing the reproducing clock signal in such a manner that the clock marks which serve to generate the reproducing clock are included in the desired recording information referred to above. Since in the first method of detection described above the clock marks recorded on the information recording layer can be detected by using light of a wavelength xcex2 differing from the light of wavelength xcex1 which serves to reproduce the reproducing layer, it does not matter whether the light of wavelength xcex1 which serves to reproduce the reproducing layer and the light of wavelength xcex2 which serves to detect the clock marks are incident simultaneously or separately. If they are incident separately, the magneto-optical recording medium may be scanned so that the xcex2 light spot precedes the xcex1.
The second aspect of the present invention provides a method of recording and reproducing a magneto-optical recording medium having an information recording layer on which information is recorded and a reproducing layer on to which information recorded on the information recording layer is transferred, comprising the steps of recording on the information recording layer recording marks corresponding to the recorded information and reproducing clock marks; and reproducing the recording marks from the magneto-optical recording medium by means of light of wavelength xcex1 while reproducing the clock marks by means of light of wavelength xcex2 (xcex1xe2x89xa0xcex2).
In the present invention, it is possible to take a clock pit recessed into the substrate as a criterion when recording clock marks on the information recording layer. By employing clock marks formed on the information recording layer in conformity with this recessed clock pit as clock marks for generating the reproducing clock, it is possible to obtain a clock which is corrected for divergence between the positions of the recording clock and the recording magnetic domain, and to generate a clock which is in exact synchrony with the recording magnetic domain recorded on the information recording layer. Consequently, through exact Synchronisation with the recording magnetic domain recorded on the information recording layer it is possible to irradiate with the reproducing light and impress the reproducing magnetic field, both modulated into pulse form, even if the magneto-optical recording medium is one designed for domain enlarged reproducing. Thus, even with still higher density it is possible to enlarge the recording magnetic domain accurately on the reproducing layer, and to read the information at high C/N.
The third aspect of the present invention provides a method of recording and reproducing a magneto-optical recording medium having an information recording layer on which information is recorded, and a reproducing layer on a substrate whereon a clock pit is formed, comprising the steps of recording clock marks and recording marks according to the information on the information recording layer in conformity with the clock pit; transferring on to the reproducing layer the clock marks recorded on the information recording layer by applying a direct-current magnetic field of the same polarity as the magnetisation of the clock marks while at the same time irradiating the magneto-optical recording medium with light, and detecting the reproducing clock signal from the clock marks transferred on to the reproducing layer; and reproducing the recording marks in accordance with the detected reproducing clock signal.
During recording in the method of recording and reproducing according to the third aspect of the present invention, information is recorded in accordance with a clock which is generated from a clock pit formed on the substrate. Clock marks in synchrony with the clock generated from the clock pit are recorded on the information recording layer alongside the recording information. During reproduction, a reproduction signal from the clock pit acts as a trigger, causing a direct-current magnetic field to be applied at the same time as irradiation with light. This makes it possible for the clock marks recorded on the recording layer to be transferred on to the reproducing layer, where the coercive force is reduced as a result of irradiation with light, and for the clock mark reproduction signals based on the clock marks to be detected from the reproducing layer. From the reproducing clock generated in accordance with the clock mark reproducing signals it is possible to determine with accuracy the position of the recording magnetic domain recorded on the information recording layer. Thus, by applying to the magneto-optical recording medium reproducing light and a reproducing magnetic field which have been modulated in accordance with this reproducing clock, it is possible to reproduce recording information from the information recording layer under optimised reproduction conditions.
The fourth aspect of the present invention provides an apparatus for recording and reproducing a magneto-optical recording medium having an information recording layer and a reproducing layer on a substrate thereon a clock pit is formed, comprising a light source for irradiating the magneto-optical recording medium with light; a magnetic field application device for applying magnetic field; a controller for controlling the light source and the magnetic field application device to record clock marks on the information recording layer; a first sensor for detecting the clock marks; and a reproducing clock generator for generating a reproducing clock in accordance with a clock mark detection signal detected by the first sensor.
In the apparatus for recording and reproducing according to the present invention, the first sensor detects the clock marks which have been recorded on the information recording layer, and the reproducing clock generator generates the reproducing clock in accordance with the clock mark detection signals. Because this reproducing clock is generated from clock marks which are recorded alongside recording marks, the positional relationship vis-a-vis the recording marks is defined accurately. Thus, by employing this reproducing clock it is possible to apply to the magneto-optical recording medium a reproducing magnetic field and reproducing light which are modulated at the desired timing in relation to the recording marks. The reproducing clock generator may be obtained by modifying the reproducing clock generator which is used in the reproduction apparatus of a conventional magneto-optical recording medium. For example, as is demonstrated in embodiment 2, it is possible to set up a phase modifying circuit which serves to ensure that the phase of the clock which is generated by detecting the light reflected from a clock pit formed on the substrate of the magneto-optical recording medium tallies with the phase of the reproducing clock marks. In other words, the magneto-optical recording medium to which the present invention pertains may also have a second sensor for detecting the clock pit signal from the clock pit; a phase comparator for detecting phase differences by comparing the phases of the clock pit signal from the second sensor and the clock mark signal from the first sensor; and a delay circuit which serves to correct the reproducing clock in accordance with the phase difference detected by the phase comparator in such a manner that the clock pit signal is in phase with the clock mark signal. In this case, the phase comparator compares the phase of the signal from the clock pit formed on the substrate with that of the signals from the clock marks recorded on the information recording layer. If there is any difference in phase between the two, a delay circuit or similar device is employed to correct the clock by the phase difference in such a manner that it tallies with the signals from the clock marks prior to generating the reproducing clock. In this manner, the reproducing clock Is synchronised with a high degree of accuracy against the information recording magnetic domain. It is therefore eminently suitable as an apparatus for reproducing MAMMOS, MSR and similar magneto-optical recording media.